The present applicant has already proposed an internal EGR control system of this kind e.g. in Patent Literature 1. In this internal EGR control system, the phase of an exhaust cam with respect to a crankshaft is changed by a variable cam phase mechanism of a hydraulically-driven type, and the lift of an exhaust valve is changed by a variable lift mechanism of an electrically-driven type, whereby the amount of internal EGR caused to remain in cylinders is controlled.
More specifically, a target internal EGR amount is calculated according to an engine speed and a demanded torque, and a target cam phase is calculated according to the calculated target internal EGR amount, whereby the variable cam phase mechanism is controlled such that the target cam phase is achieved. Further, according to the target internal EGR amount, the valve-closing timing of the exhaust valve is calculated. Then, according to the calculated valve-closing timing of the exhaust valve and the actual cam phase, a target lift is calculated, whereby the variable lift mechanism is controlled such that the target lift is achieved. As a result, even when the actual cam phase is late in converging to the target cam phase due to delay in the response of the variable cam phase mechanism, the internal EGR amount is controlled such that it becomes equal to the target internal EGR amount.
However, in the above-described internal EGR control system, the target internal EGR amount is calculated according to the engine speed and the demanded torque. Therefore, when the temperature of the wall surface of each cylinder is low during e.g. warm-up operation, loss of heat from combustion gases to the wall surface of each cylinder increases, so that the temperature of working medium before combustion in the cylinders lowers, which sometimes makes the combustion state unstable even if the internal EGR amount is the same. There is room for improvement in this point.
The present invention has been made to provide a solution to the above-described problem, and an object thereof is to provide an internal EGR control system for an internal combustion engine, which is capable of ensuring an excellent combustion state by properly controlling an internal EGR amount.
[Patent Literature 1]
Japanese Laid-Open Patent Publication (Kokai) No. 2008-106654